The present invention relates to a method for detecting a fire condition in a monitored region, and particularly to such a method effective at relatively long ranges and/or with relatively small fires.
One of the problems in detecting fire conditions, particularly at long ranges or of small fires, is the high false alarm rate. Thus, the range of detection can be increased by increasing the sensitivity of the system, e.g., by appropriately setting the amplification level and/or the threshold level. However, this increase in sensitivity also tends to increase the false alarm rate caused by spurious radiation sources, such as sunlight, artificial light, welding, electrical heaters, ovens, etc., or by other sources of noise. Such spurious radiation sources might not be large enough to activate short-range detectors, but may be large enough to activate detectors whose sensitivity has been increased to increase the range. A false alarm may result in a costly discharge of the fire extinguisher; and if the fire extinguisher is of the type requiring replacement before reuse, the false alarm may disable the fire extinguisher system until it has been replaced or recharged.
A number of attempts have been made for increasing the range of a fire detector system without substantially increasing the false alarm rate. Some described systems utilize two sensors in different spectrum ranges, as illustrated in U.S. Pat. Nos. 3,653,016, 3,665,440, 3,825,754, 3,931,521, 4,639,598 and 4,983,853. Other described systems utilize an AC coupling and a level ratio test, as illustrated in U.S. Pat. No. 4,455,487. In another proposed system, the detector examines the frequency characteristics of monitored signals produced by a sensor in order to distinguish between fire-produced radiation and spurious radiation.